1. Technical field
This invention is in the field of magnetic sensors which detect a magnetic field or magnetic field change and find use in linear or rotary motion detection, electrical current sensing, linear or rotary position sensing, magnetic imaging, magnetic recording read heads, magnetic recording media as well as general magnetic field sensing.
2. Background of the Invention
Because of the many applications of magnetic field sensors, there is a long history of technical development of materials and means to measure fields of various strengths. No one sensor can perform every function well. Factors such as size, weight, power consumption, and cost should be minimized by a field sensor. Sensitivity, linearity, bias, stability, reliability, and operating temperature and frequency range are factors that should be optimized. As with any instrument, it is usually difficult to achieve all of these characteristics in one device. The most common magnetic sensors used in a variety of applications are the Hall effect sensor and the variable reluctance coil. The drawback of variable reluctance devices is that they generate signals proportional in size to the time rate of change of magnetic flux. The signal size therefore decreases with decreasing speed, and below a certain flux change rate, the signal disappears into the noise. Hall effect devices generate a very small raw signal because of low field sensitivities (0.5.about.5 mV/100 Oe applied field). This mandates signal conditioning, and requires that a certain minimum field be available for device operation.
The concept in combining the magnetostrictive materials and piezoelectric layers for highly sensitive magnetometer was first introduced by Mars D. Mermelstein in 1986. In his patents U.S. Pat. No. 4,769,599 and U.S. Pat. No. 5,130,654 a magnetometer is disclosed as a device using a piezoelectric resonator to create a standing wave in a sensing stress wave in the sensing magnetostrictive ribbon and using a pickup coil to read out the electromotive force. A minimum detectable field gradient of 7.7 pT/cm Hz was achieved in this device by using a differential amplifier technique. In October 1997, a device called piezomagnetometer was patented by Walter N. Pondney. In that device a stack of 201 alternating piezoelectric and magnetostrictive layers is used, in which 100 pairs of piezoelectric-magnetostrictive layer capacitors are connected electrically in parallel to increase the charge storage by raising the effective capacitor plate area. The device requires multiple layers that are placed in a permanent biasing magnetic field normal to the layer surface. A resolution as high as 1 pT/cm-Hz is achieved.